This invention relates generally to processes for producing ceramic products. More particularly, this invention relates to a process for producing a ceramic product wherein a ceramic material is worked by a machining method by which the material can be contoured into a complex or intricate shape, and the thus-formed article is fired.
Ceramic products comprising fired articles of ceramic materials such as oxides, nitrides, carbides, borides and silicides have excellent thermal, chemical, mechanical and electromagnetical characteristics and therefore are used in a variety of ways. Moreover, the utilization fields thereof are being continually enlarged. However, these ceramic products are generally hard and brittle, and therefore it is difficult to form them into desired shapes after firing or sintering. Thus, such ceramic products are accompanied by a processing drawback. Therefore, the ceramic material powder is formed into a desired shape before firing and then fired to obtain a ceramic product having any of a variety of shapes.
Hitherto, cast molding, rubber pressing, injection molding, hot pressing and the like have been used as methods of molding ceramic materials such as oxides. However, the molding methods for producing products having a complex shape with high precision are restricted to cast molding or casting, injection molding and the like.
In the case of casting, a blend of ceramic starting materials is mixed with a suitable medium to prepare a slip or slurry, and this slip is cast into a mold having a desired shape while imparting fluidity to the slip to mold the ceramic material therein. However, in the case of such a forming method, the interior of the formed product tends to be heterogeneous, the casting mold used is expensive, and the fabrication thereof is time consuming. Therefore, in the case of production of products of numerous types each in a small amount, such a process is disadvantageous in that the production cost is very high.
In the case of injection molding, a small amount of a plasticizer and a releasing agent are blended into the ceramic starting material powder, and the resulting mixture is heated in a heating cylinder to impart plasticity thereto and injected into a mold by a pressure plunger to mold the mixture. However, such a molding method presents the following problems. Internal stress is liable to be generated in the workpiece during molding, and cracking may occur during the subsequent firing. Further, as with casting, the mold is expensive and the fabrication thereof is time consuming. Therefore, in the case of production of products of various types in small amounts, such an injection molding is unsuitable.
If ceramics could be worked by a machining technique used in the forming of metals, wood and the like, it would be possible to carry out numerous-type, small-quantity production of the ceramic products. The prior art process of machining ceramics comprises forming (i.e., primary forming) a ceramic starting material powder by any of various methods, semi-drying, drying or biscuiting the formed structure and thereafter carrying out turning or milling by cutting tools provided with abrasion resistant materials such as tungsten carbide and sintered alumina.
While this prior art process of machining ceramics can be applied in forming products having simple shapes with features such as holes, grooves, planes and curved surfaces via rotational machining, the process as described above has not been successfully used in forming products having complex shapes such as turbine rotors having complex and thin portions (blades). The reasons for this are thought to be as follows. As described hereinbefore, in the case of the prior art machining process, the strength of the product of primary forming is low, and therefore the presence of thin formed portions inevitably results in a loss of yield due to breakage. Furthermore, the prior art process of machining ceramics as described above is unsuitable for precision forming.